The invention is in the field of arrangements providing signal representative of at least two images to be displayed superimposed on a screen.
For instance, in a VCR a composite video signal is read/recorded from/on a tape, by magnetic reading and writing heads, amplified and then sent to a video integrated circuit that processes the amplified signal received from the heads.
Superimposed , in a known way, to the first image, there could be a second image, e.g. what is known as an On Screen Display (OSD). Such an OSD is most of the time a set of conventional signs or characters which are generated in a devoted circuit (OSD generator) according to control issued by a user, for instance with a remote control. The OSD is generated most of time in a dedicated circuit for instance a microcontroller that receives the orders from the user and displays an image in accordance with said control and a resident software.
The OSD is generated independently from the video signal and must appear at the same place, from an image to the next. That means that said OSD must be synchronised with the main display. In order to synchronise the OSD signal with the displayed image, the OSD devoted circuit receives the horizontal and vertical synchronisation pulses (Hsync and Vsync) from a synchro separator circuit that receives the composite video signal and extracts from it said horizontal and vertical synchronisation pulses. Then by counting beats of a time counter starting from one of the horizontal synchronisation pulses that appears after a determined number of other horizontal synchronisation pulses, starting from each vertical synchronisation pulse, it is possible to find out the instant at which to start the OSD. The synchronisation signal extracted by the synchro separator circuit is known as composite synchronisation signal (Csync). Csync comprises Hsync (horizontal synchronisation) and Vsync (vertical synchronisation).
During Electronic-to-Electronic mode (a mode often said Exe2x80x94E mode, in which recording is not performed, but a signal from a source for instance a video camera, goes through the VTR circuits and can be viewed in a TV monitor) or playback modes, the Csync signal is noise-free. Problem occurs during trick modes such as pause or fast forward or rewind. The sync-separator circuit exhibit xe2x80x9cspuriousxe2x80x9d vertical sync pulses, especially when the playback Frequency Modulated (PB-FM) signal is weak. Said spurious signal appears in the time period that elapses between two frames.
Since the OSD dedicated circuit is using this signal as a reference for OSD generation, the OSD would then jitter vertically. This is highly undesirable, especially during assembly recording when the pause mode is used to view for instance the start and end of each recording sequence, and at the same time, the OSD must be clearly readable.
The purpose of the invention is to solve this problem of vertical jitter that appears each time that a trick mode is used. A trick mode is a mode in which the speed of the tape is different from the normal play speed, for instance pause or fast rewind or forward.
To get around this problem, according to the invention a switching circuit is implemented to supply the OSD dedicated circuit, which is often a micro controller, with the normal vertical synchronisation signal coming from the synchro-separator circuit during Exe2x80x94E and playback modes, but switch to PV (pseudo-vertical) signal during trick modes such as pause, slow-motion, or search modes. This PV signal is generated during trick modes only. For a VCR an already existing circuit can be used. It is very stable because it is derived from the DFF (Drum flip-flop), which is constructed using information from the drum pickup. It does not depend on the strength of the playback FM or the performance of the synchro-separator circuit.
To sum up the invention is about an arrangement comprising:
means for providing a signal representative of a first image to a screen, said signal including a vertical synchronisation signal,
means for providing a pseudo-vertical synchronisation signal,
generator means for generating a second image, said generator means having at least an input for receiving a signal to synchronise the second image with the first image,
control means having an output indicating the mode in which the arrangement is functioning,
selector means for selectively providing said vertical synchronisation signal or said pseudo vertical synchronisation signal to said input depending on the output of the control means.
According to preferred embodiments:
the generator means comprises an On Screen Display generator,
the means for providing the signal representative of the first image includes a rotary drum fitted with at least a pair of magnetic heads for reading a magnetic tape,
the means for providing a pseudo vertical synchronisation signal comprise a control circuit elaborating the pseudo-vertical synchronisation signal on the basis of control pulses generated by the rotary drum,
the arrangement further includes separator means for taking the vertical synchronisation signal out of the signal representative of the first image,
the selector means are in the form of a switch with a first input connected to the separator means, with a second input connected to the control circuit with a thirds input connected to the control means and with an output connected to the input of the generator means.